Fiber-optic devices for receiving and transmitting video and audio.
Video signal transmission in video surveillance systems is one of the priority tasks. But quite often, in order to obtain complete information about what is happening at the surveillance site and to make a timely and adequate decision, the video image needs audio accompaniment. And the task of transmitting an audio signal (and an audio signal together with video) over significant distances can be handled by systems built on the basis of devices for transmitting signals via optical fiber. We familiarized ourselves with the basic concepts associated with video signal transmission in previous issues (No. 1–3 – 2010). The purpose of this review is to answer the main questions that arise when selecting equipment for transmitting video and audio signals.
Does the fiber type (single- or multimode) affect the quality of transmitted video and audio signals?
The fiber type in no way affects the quality of transmission of any signals. The choice of fiber type is only important when calculating the distance over which the signal will be transmitted.
Which audio signal (analog or digital) is better to transmit via fiber optics?
As in the case of transmitting a single video signal, when it is necessary to transmit video and audio signals simultaneously over long distances, it is better to use digital equipment.
Does the number of channels affect the quality of the transmitted signals, or does the transmission of other types of signals (telemetry signals, contact status, etc.) via the same fiber?
The number of different types of signals transmitted over a single optical fiber does not affect the transmission quality in any way. The number of transmitted signals is taken into account by the equipment manufacturer when choosing the multiplexing method (WDM, CWDM, etc.).
What are the main parameters that need to be considered when selecting equipment for transmitting video and audio signals?
Many technical characteristics specified by manufacturers are standard for devices of a certain purpose. In addition to the main characteristics (optical budget, operating temperature, modular or rack-mount design, etc.), when choosing equipment, it is necessary to pay attention to the characteristics that describe the features of signal transmission of a particular type. In previous issues, we considered the characteristics that need to be taken into account when selecting equipment that transmits a video signal. In addition to them, when choosing devices that simultaneously transmit video and audio signals, we recommend paying attention to:
sampling bit depth when digitizing an audio signal. CCTV systems use values in the range of 16–24 bits. The higher the bit depth, the better the quality of the digitized sound;
frequency range (20–20,000 Hz is already considered the norm for sound transmission and reproduction);
signal-to-noise ratio. Values in the range of 45–75 dB are often encountered. The higher this value, the cleaner and better the sound.
The following issues of the magazine will cover devices that transmit video signals and telemetry signals via fiber optics.